The invention relates to a load-controllable engine braking method which comprises a process for preventing fluid pressure on an auxiliary piston in an engine braking device to a main piston. The engine braking device comprises a pressure-limiting mechanism, a pressure-limiting valve of the pressure-limiting mechanism is arranged between a main piston hole and an auxiliary piston hole, and the pressure-limiting valve is kept to be in the normal-open state through a pressure-limiting spring; a cam drives the main piston, the main piston drives the auxiliary piston through hydraulic pressure, an air valve of an engine is opened through the auxiliary piston and applies counter-acting force to the auxiliary piston, the pressure of fluid in the auxiliary piston hole is increased, pretightening force of a pressure-limiting spring on the pressure-limiting valve is overcome, the pressure-limiting valve is closed, and pressure of fluid on the auxiliary piston is prevented from being transmitted to the main piston. The pressure of the fluid on the auxiliary piston is transmitted to a stopping mechanism fixed to the engine rather than the cam of the engine. A box of the engine braking device comprises a valve bridge and a rocker of the engine. By means of the load-controllable engine braking method, loads on an engine air valve driving chain can be controlled, and the reliability and durability of the engine are increased.